restic backup ... -o "rclone.args=serve restic --stdio --tpslimit n --other-flag[s]" ...
or set rclone
environment variables, below example for heavy throttling:
export RCLONE_TPSLIMIT = "5"
export RCLONE_TPSLIMIT_BURST = "0"
export RCLONE_TRANSFERS = "1"
export RCLONE_CHECKERS = "1"
In case of GoogleDrive make sure to configure your own client_id as per rclone docs